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Date:   Mon, 25 Mar 2019 22:49:59 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Daniel Borkmann <daniel@iogearbox.net>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        syzbot <syzbot+7a8ba368b47fdefca61e@syzkaller.appspotmail.com>,
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Subject: Re: KASAN: use-after-free Read in path_lookupat
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On Mon, Mar 25, 2019 at 11:04:53PM +0100, Daniel Borkmann wrote:

> +static void bpf_destroy_inode_deferred(struct rcu_head *head)
> +{
> +	struct inode *inode = container_of(head, struct inode, i_rcu);
> +	enum bpf_type type;
> +
> +	if (S_ISLNK(inode->i_mode))
> +		kfree(inode->i_link);
> +	if (!bpf_inode_type(inode, &type))
> +		bpf_any_put(inode->i_private, type);
> +	free_inode_nonrcu(inode);
> +}

Umm...  Can bpf_any_put() can be called in that context?  It's going
to run in softirq; if that's OK (I'm not familiar enough with BPF guts
to tell), sure, that's probably the best approach.

Note, BTW, that it's obviously -stable fodder, so any splitting
of ->destroy_inode() would live on top of that fix.